28 



THE FE.\THERED TltlBES. 



The eggs of tlie barn-door fowl arc white ; those of the nightingale of an olive-brown ; 

 and. other great differences might easily be mentioned. As there are certain glands 

 which secrete the general ground colour of a shell, so there are, most probably, others 

 among them, which supply the additional colouring matter. It has been proved that 

 the one only may remain, while the other is removed. Thus, the egg of a song-thrush 



FIG 21.— SECTION OF A UEn's KGG. 



has a bright blue ground colour, with irregular spots and blotches of black ; but the 

 latter have been preserved from the action of acid which has entirelj' discoloured the 

 former ; and this is only one instance among many, which might readily be given. 



The egg of a bird is beautifully adapted to the process it has to undergo. Externally, 

 this is the case : for its oval form jDcrmits a greater part of its surface to be in contact 

 with the warm body of the parent, than any other with which we arc acquainted ; even 

 that which approaches it most nearly — the form of a gloljc. 



Internally, arrangements are equally perfect. For in an egg ready for incubation, a 

 small spot may be observed on the yolk (fig. 21, 1), which, when examined, is appa- 

 rently a vesicle, or bag, containing a fluid matter. In this then, the embryo swims, and 

 as it is necestsary that it should be, not at the cold bottom of the nest, but close to the 

 breast of tlic mother-bird, an exquisite provision is easily discoverable. 



The yolk of the egg is in fact a globe, and at its surface the little A-esiclc containing the 

 ombryo is precisely placed. Nor is this all. For were its axi.s thiough the centre, It 

 ■wouli not move with the cluinge in the position of the egg ; but the axis is ])hued below 

 the centre, so that it must turn round with every such change, wlu-tlui- (he globe be 

 heavier or lighter, than the surrounding white. Were it heavier, tlu' embryo would fall 

 to the lower part of the slicll : but it is lightii, imd ihircfore always rises to t lie upper 

 surface. 



Such important results are secured by the most a))])r(ipriat(- means. ( )n examination, 

 two dense, lough cords ap])eai' i'2). eacli of tlinn being siroiigly attaclicd to tiie yolk-bag. 

 They then pass througli the wliite, and are connected «ith tlie iinier lining membrane at 



